Ranging system using phase detection

ABSTRACT

A modulated alternating waveform produced by an oscillator is transmitted to a target and a receiver receives reflections of the waveform from the target. The waveform of the modulation also is applied through a voltage controlled phase shifter and squaring amplifier to a synchronous detector coupled to the receiver. In the synchronous detector, the signal and any spurious noise from the receiver is multiplied by the waveform from the squaring amplifier in a manner that cancels out all spurious noise. A DC voltage representative of the phase shift produced in the modulation of the received signal by the distance from transmitter to target to receiver is applied to an integrator. Integrator output is applied to the voltage controlled phase shifter and is representative of the distance traveled by the waveform.

United States Patent [151 3,680,092 Scott 1 July 25, 1972 [54] RANGINGSYSTEM USING PHASE Primary ExaminerT. H. Tubbesing DETECTIONAttorney-John R. Faulkner and Glenn S. Arendsen [57] ABSTRACT Amodulated alternating waveform produced by an oscillator is transmittedto a target and a receiver receives reflections of the waveform from thetarget. The waveform of the modulation also is applied through a voltagecontrolled phase shifter and squaring amplifier to a synchronousdetector coupled to the receiver. In the synchronous detector, thesignal and any spurious noise from the receiver is multiplied by thewaveform from the squaring amplifier in a manner that cancels out allspurious noise. A DC voltage representative of the phase shift producedin the modulation of the received signal by the distance fromtransmitter to target to receiver is applied to an integrator.Integrator output is applied to the voltage controlled phase shifter andis representative of the distance traveled by the waveform.

21 Claims, 5 Drawing figures LTRANsmTTERHsQuAR MscmLAiOF] I l l l PHASESHIFTER l sum 1 or 3 5 I 1 J TRANSMITTER SQUARER OSCILLATOR SQUARER|-VOLTAGE CONTROLLED I L I PHASE SHIFTER I 24 I 22 a- 2 8 I I RECEWER g qg gg INTEGRATOR I L 4 i 4 LTRANSMITTER HSQUARER HosclLLAToR] r j/JoPHASER RAMP I GENERATOR I i I 24/5QUARER COMPARATOR LOW PASS Z6 FILTER I/6 I SYNCHPONOUS RECEIVER I DETECTOR INTEIGRATOR I ATT OR NE RANGINGSYSTEM USING PHASE DETECTION BACKGROUND OF THE INVENTION Highway safetyhas become a major consideration of modern society, and forecasts of thegreatly increasing number of vehicles expected on the highways in thefuture indicate that the safety problem soon will increase by severalorders of magnitude. Numerous techniques have been proposed recently forcontrolling a vehicle according to the amount of its available headway,i.e., the distance from the vehicle to an object located in its path.Essential to such techniques is a ranging system for measuring theavailable headway under the conditions existing on the highways.

Early ranging systems typically used the Doppler effect to determine theclosing rate of a vehicle on a forwardly located object and used theclosing rate to estimate available headway. Later systems measuredintensity changes in electromagnetic waves resulting from the distancetraveled by the waves. More sophisticated systems have attempted toutilize the phase shift of a wave that has been transmitted toward andreflected from a forwardly located object. The excessive amounts ofspurious electrical noise generated by vehicle ignition equipment, radioand television transmission devices, high tension power transmissionlines and transformers and the many other electrical and electronicdevices found on and near modern highways renders proper detection anddiscrimination of the reflected signals extremely difficult.Additionally, prior art ranging systems generally are unable todistinguish with any significant reliability between its own reflectedsignal and the signal of similar systems on other vehicles; this createsserious difficulties not only when two vehicles are meeting but alsowhen one vehicle is passing another.

SUMMARY OF THE INVENTION This invention provides a ranging system fordetermining the distance to an object that is capable of distinguishingreflections of its own transmitted wave from virtually all spuriousnoises. The system is useful particularly on automotive vehicles thatoperate in high noise environments, for it is capable of detecting anddiscriminating its reflections from very low signal to noise ratios,typically as low as 1:100. A DC voltage is produced by the rangingsystem that is directly proportional to the distance traveled by thetransmitted wave and is useful readily in electrically actuated vehiclebraking and accelerating mechanisms. The system also produces a voltagerepresentative of the rate of change of that distance.

All components of the system are mounted on a single vehicle orinstallation since the system does not require any cooperatingcomponents in other vehicles.

In the system, a signal transmitter is mounted where it can transmit acarrier signal toward a reflecting target. The carrier signal preferablyhas a frequency between about 1,000 and 330,000 gigahertz. A modulatingoscillator modulates the carrier signal with a waveform preferablyhaving a frequency between about kilohertz and 10 megahertz. A signalreceiver is located where it can receive reflections of the modulatedcarrier signal from the reflecting target. Ranging systems forautomotive vehicles typically have the transmitter and receiver locatedin the grille or on top of the instrument panel where the devices havean unobstructed view of the headway path.

A phase detection circuit that not only separates the modulation on thereceived carrier signal from spurious noises but also determines thephase shift caused by the distance traveled by the modulated carriersignal in moving from the signal transmitter to the target and back tothe signal receiver is located in the vehicle. Included in the phasedetection circuit is a synchronous detector connected to the modulatingoscillator and the signal receiver. The detector operates at thefrequency of the modulation so it will detect only received signalshaving the same frequency, and it produces a DC voltage representativeof the phase difference between the modulation on the received signaland the original modulation. An

integrator has its input connected to the detector and its outputconnected to a voltage controlled phase shifter. The phase shifter alsois connected to the modulating oscillator and its output is applied tothe synchronous detector through a squaring device that squares thesignal along the time scale. Thus the phase detection circuit comprisesa synchronous detector, an integrator, a voltage controlled phaseshifter and a squaring device connected in a closed loop, with thedetector also connected to the receiver and the phase shifter alsoconnected to the modulating oscillator. An external output terminal isconnected between the integrator and the phase shifter.

Operation takes place in the following manner. An alternating waveformproduced by the oscillator is applied through a squaring or dividingdevice that reduces the frequency by one half to the transmitter wherethe waveform is converted into a sine wave, applied as modulation to acarrier signal, and trans mitted toward the target. Oscillator outputalso is applied to the voltage controlled phase shifter which shifts thephase thereof according to the voltage received from the integrator.Phase shifter output is applied to a second squaring or dividing devicewhere the signal is squared along the time axis and reduced in frequencyby one half. The squared waveform is applied to the synchronous detectoralong with the target reflections and spurious noise picked up by thereceiver.

In the detector, receiver output is multiplied by the squared waveformfrom the squaring device. Any receiver output having a frequency equalto the squared waveform frequency produces a net DC voltage proportionalto the amount of the phase difference between that frequency and thesquared waveform while all other signals in the receiver output producean alternating wave having a net DC voltage of zero. Any DC voltage isapplied to the integrator, which multiplies the DC voltage by time andapplies the product to the voltage controlled phase shifter. If thephase difference between the inputs to the detector is exactly detectoroutput is zero and integrator output can be zero. As the phasedifference varies from 90, detector output changes positively ornegatively depending on the direction of the phase shift in the receivedsignal. The integrator then produces a DC voltage that is the timeintegral of the detector output, and this voltage operates in the phaseshifter to shift the squared waveform thereof to the point wheredetector output again returns to zero. The integrator maintains this DCvoltage until another phase shift occurs, in which case the samesequence produces another integrator output voltage that operates in thesame manner to return detector output to zero. Integrator output voltagethus is related directly to the phase shift and in turn to the rangebetween the transmitter-receiver and the target. In addition, the DCvoltage from the detector is directly related to the rate of change ofthe range.

In a preferred embodiment, the voltage controlled phase shifter includesa ramp generator that produces a linear ramp function having a frequencyequal to the oscillator frequency. A comparator receives the rampfunction and the DC voltage from the integrator and produces a waveformhaving a positive portion whenever the ramp function exceeds theintegrator voltage and a negative portion whenever the ramp function isless than the integrator voltage. Integrator output thus shifts thepoint in time at which the waveform begins its positive portion. Thesquaring amplifier squares the waveform along the time axis by reactingonly to positive values of the positive going pulse of each cycle, andapplies the resulting waveform to the detector as described above.

The range scale of the ramp generator system is limited as a practicalmatter to ranges corresponding to about 90 of phase shift since thesystem cannot discriminate between the waveforms generated on succeedingramps and a finite time is necessary for each ramp to decay. This rangecan be virtually tripled by including in the transmitter circuit amechanism for also shifting the phase of the transmitted wave accordingto the detected phase difference. To accomplish this, a secondcomparator is connected into the transmitter circuit and is controlledby the integrator output so integrator output shifts one comparator inone direction and the other comparator in the other direction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a rangingsystem of this invention showing the phase detection loop circuit madeup of the voltage controlled phase shifter, squaring amplifier,synchronous detector and integrator.

1 FIG. 2 is a block diagram of an improved version of the invention inwhich a ramp generator is used to convert oscillator output into a rampfunction.

FIG. 3 is a block diagram showing a system in which phase changes act intwo separate comparators, one shifting the point on the ramp function atwhich the transmitted wave is sent and the other shifting the point onthe ramp function at which the detecting signal waveform is applied tothe synchronous detector.

FIG. 4 is a diagram showing the relative phases of the waveformsproduced by certain devices of the FIG. 2 system and includes thewaveforms present during a phase change.

FIG. 5 is a diagram similar to that of FIG. 4 but showing the relativewaveform phases in the FIG. 3 system.

DETAILED DESCRIPTION Referring to FIG. 1, the modulating oscillator forthe system of this invention is indicated by numeral 10. Oscillatorpreferably produces a square waveform but any other waveform also can beused. The oscillator operates at twice the frequency desired in thetransmitted modulation and it is connected through a squarer 12 to thesignal transmitter 14. Squarer 12 divides the waveform frequency by two;typically squarer 12 performs its division by producing a square wave inresponse only to rising positive voltages in the waveform produced byoscillator 10. The squarer also, however, can respond to decliningvalues of negative voltage in the waveform of oscillator 10 or to anypredetermined value thereof. Transmitter 14 receives the squaredwaveform from squarer 12, converts the waveform into a sine wave ifnecessary and uses the sine wave to modulate a carrier signal receivedfrom a carrier oscillator (not shown). The transmitter then transmitsthe modulated carrier signal forwardly of a vehicle toward a target 16.

A receiver 18 is mounted on the front of a vehicle where it will receivereflections of the modulated carrier signal from target 16. The receiveris connected to a phase detection circuit enclosed by dashed line 20.Phase detection circuit 20 includes a voltage control phase shifter 22connected in a closed loop configuration with a squarer 24, asynchronous detector 26, and an integrator 28. Oscillator 10 isconnected to phase shifter 22 and receiver 18 is connected tosynchronous detector 26.

Turning to FIG. 2, the voltage controlled phase shifter 22 preferably ismade up of a ramp generator 30 connected to oscillator 10 and acomparator 32 that receives the output of the ramp generator. Comparator32 also receives the output of the integrator 28 via a low pass filter34. The output of comparator 32 is applied to squarer 24. Comparatorinput is applied to an external terminal V and integrator input isapplied to an external terminal V.

The operation of the FIG. 2 circuit can be understood more readily byreferring also to FIG. 4. As indicated in FIG. 4, oscillator 10continues to produce an alternating waveform 36. The frequency of thiswaveform is halved and converted into a sine wave 38 that is transmittedas modulation by transmitter 14.

Ramp generator 30 responds to the square waveform 36 produced byoscillator 10 to produce a sawtooth waveform 40, each cycle of whichincludes a linearly increasing ramp 42 terminated by a rapidlydecreasing portion 44. Ramp 42 rises linearly from zero when waveform 36begins its positive portion to a predetermined positive voltage that isreached when waveform 36 completes its positive portion, declinesrapidly along portion 44 to a negative voltage having substantially thesame value as the predetermined positive voltage, and then begins risingalong the succeeding ramp 42 while waveform 36 completes its negativeportion. Ramp 42 passes through zero when waveform 36 passes throughzero.

Waveform 40 is one of the inputs to comparator 32 which has as itssecond input a DC voltage 46 from integrator 28. Comparator 32 producesa square waveform 48 that begins a positive portion whenever the voltageof ramp 42 exceeds the DC voltage 46 from the integrator. Each positiveportion of waveform 48 lasts until waveform 40 becomes negative at whichpoint waveform 48 also produces a negative portion that lasts until thevoltage of ramp 42 rises above DC voltage 46. When integrator output 46is zero, comparator output is a symmetrical square waveform 48corresponding to the waveform 36 of oscillator 10, but as will be seen,any changes in the value of DC voltage 46 produce variations in theshape and phase of waveform 48.

Squarer 24 receives the output waveform 48 from comparator 32 andresponds only to positive going pulses in waveform 48 to produce asquare waveform 50. Squarer 24 can be a binary flip flop that becomespositive at one positive going portion of waveform 48, negative at thenext positive going portion of waveform 48, positive at the nextpositive going portion of waveform 48, etc. Waveform 50 has a frequencyequal to that of transmitted modulation 38 and a phase determined by thephase of comparator 32 output. The phase of waveform 50 initially is 180from that of transmitted waveform 38 when integrator output is zero asshown in the portion of FIG. 4 to the left of dashed vertical line 56.Other initial phase arrangements can be used and a phaser 25 isconnected between squarer 24 and squarer 12 to provide desired initialphasing. Waveform 50 is applied to synchronous detector 26.

Detector 26 also receives the output of receiver 18, which outputincludes a waveform 52 corresponding in frequency and shape to thetransmitted modulation waveform 38, assuming that the transmittedwaveform has struck a target 16 and has been reflected back towardreceiver 18. In accordance with well known principles, the phase ofwaveform 52 will be delayed from the phase of transmitted waveform 38 byan amount directly proportional to the distance traveled by themodulation from the transmitter 14 to the target 16 and then to receiver18. This phase delay is used to determine the distance between a vehiclecarrying transmitter 14 and receiver 18 and an object 16 located in theheadway path of the vehicle. In FIG. 4, the initial phase delay isassumed to be and waveform 52 thus lags waveform 38 by 90.

As pointed out above, detector 26 multiples the output of receiver 18 bywave 50. If the received waveform S2 lags transmitted waveform 38 by 90as shown in the left portion of FIG. 4, the detector produces an outputwaveform 54 as shown that has a net DC voltage of zero. The net DCoutput of detector 26 is applied to integrator 28 which applies the timeintegrated product of the output to comparator 32 as DC voltage 46. Solong as received waveform 52 lags the transmitted waveform 38 by 90,integrator output 46 remains at zero.

At the point in time represented by vertical dashed line 56, it isassumed that the distance between the vehicle and the target increases,which of course results in an increased delay in the phase of receivedwaveform 52. This increased delay takes place gradually but is shown forillustrative purposes in FIG. 4 as an immediate lag of about 45 thattakes place in the space between vertical dashed lines 56 and 58.

When the delayed waveform 52 is multiplied by waveform 50 in detector26, the detector output assumes the modified waveform 54 shown on theright side of vertical line 58. Waveform 54 has a definite net positiveDC value which integrator 28 converts into a steadily rising DC voltage46. Since comparator 32 is triggered only when the voltage of rampfunction 42 is more positive than the voltage 46 of integrator 28, therising integrator voltage 46' delays the triggering point of thecomparator to produce the modified comparator output waveform 48. Thisin turn delays the phase of the waveform produced by squarer 24 asindicated at numeral 50'. DC voltage 46' from integrator 28 continues torise until the delay in the squarer output catches up with the delay inreceived waveform 52, which is illustrated in FIG. 4 as occurring atdashed vertical line 60. When squarer output 52 again is exactly 90 outof phase with received waveform 52, detector 26 again produces itssymmetrical waveform 54 which has a net DC value of zero. The DC outputof integrator 28 then stops increasing but maintains its increased level46" to maintain the 90 relationship between squarer output and receivedwaveform 52. Received waveform 52 now lags the transmitted waveform 38by about 135 and DC voltage 46" from integrator 28 is representative ofthis additional phase lag.

Similar operation maintains the DC voltage from the integratorrepresentative of the distance between the transmitter-receiver and thetarget. Thus, further increases in the distance produce a greater delayin received wave 52, which increases voltage 46 still further tocompensate therefor. If the distance decreases, the phase delay inreceived wave 52 also decreases and detector 26 begins producing a netnegative DC voltage that reduces the output of integrator 28 and canmake that output negative in value. Voltage 46 and its varied voltages46' and 46" appear at external terminal V where values thereof alwaysrepresent range. The net DC voltage 54, 54' of the detector outputappears at external terminal V' where values thereof always representthe rate of change of the range.

Any spurious noises or other waveforms differing even slightly infrequency from received waveform 52 that are picked up by receiver 18produce alternating waves having a net value of zero when multiplied indetector 26 by waveform 50. Spurious noises differ significantly fromthe frequency of waveform 52 produce a net DC voltage of zero within avery few cycles, which occurs so rapidly that integrator 28 does notreact thereto. Received waveforms differing only slightly from waveform52, however, might produce an AC signal that is picked up and integratedby integrator 28. Low pass filter 34 removes any such ripples from theoutput of the integrator to prevent an undesired reaction of comparator32 to such signals.

The FIG. 3 circuit is similar to the circuit shown in FIG. 2 butincludes additional components that effectively triple the range of thesystem. Each of these additional components is described below. Ifnecessary, a driver 13 can be connected between squarer 12 andtransmitter 14 and an amplifier 19 can be connected between receiver 18and detector 26 to increase the ranging ability of the transmitter andreceiver.

A transmitting comparator 62 is connected between ramp generator 30 andtransmitting squarer l2, and an inverter 64 connects low pass filter 34to comparator 62. Low pass filter 34 also is connected to the detectingcomparator 32 as in FIG. 2.

FIG. 5 illustrates the operation of FIG. 3. Phase detection circuitoperates in a manner identical to that of FIG. 2 as described above. Theoutput of integrator 28 is applied not only to comparator 32 but also toinverter 64 which in turn applies the inverted integrator DC voltage(represented by numeral 66) to transmitting comparator 62. Transmittingcomparator 62 thus receives the waveform 40 from ramp generator and theinverted DC voltage 66 from integrator 28.

The transmitting comparator 62 functions like detecting comparator 32 inthat it produces a waveform 68 that begins a positive portion each timethe voltage of ramp 42 exceeds the DC voltage 66 in comparator 62 andproduces a negative portion whenever the voltage of the ramp function isless than the inverted voltage 66. Transmitting squarer 12 responds onlyto positive going pulses in waveform 68 to produce a square waveform 70.Driver 13 converts square waveform 70 into an inverted sine wavefunction 72 that is transmitted toward the target by transmitter 14.

If an increasing distance between transmitter-receiver and target 16produces a greater lag in the phase of received wave 52 as illustratedat dashed vertical line 56 in FIG. 5, the resultthe received waveform52, the phase of the transmitted modulation is advanced, which producesa corresponding advance in the phase of received waveform 52. Phasechanges that move the positive integrator voltage 46 all the way to thetop of ramp 42 and simultaneously move the inverted voltage 66 all theway to the bottom of ramp 42 thus can be measured. Since the amount oftime necessary for ramp delay remains about the same in the FIG. 3system as in the FIG. 2 system, about 270 are available for rangemeasuring, which is 3 times the amount available in the FIG. 2 system.

When the systems are used in production vehicles, a large number ofslightly different modulation frequencies are provided. Because of thehigh degree of discrimination inherent in the system, interferencebetween similar systems on different vehicles is minimal. Transmittersand receivers also are designed to transmit and receive narrow beamsonly.

Thus the invention provides a ranging system that is highly accurate andvery capable of discriminating its signals from spurious noise. Thesystem produces a DC voltage directly representative of range, and alsoproduces a DC voltage representative of the rate at which the range ischanging. Such DC voltages are useful directly to control various itemsof vehicle equipment.

I claim:

1. An electronic ranging system comprising signal transmitting means fordirecting a carrier signal toward a reflecting target,

modulating means for modulating said carrier signal,

signal receiving means for receiving reflections of the modulatedcarrier signal, and

phase detection means for determining the phase shift resulting from thedistance traveled by said modulated carrier signal from said signaltransmitting means to said signal receiving means, said phase detectionmeans including a voltage controlled phase shifting means connected tothe modulating means, synchronous detector means connected to said phaseshifting means and said signal receiving means, said detector meansoperating at the frequency of the modulation to detect only receivedsignals having the same modulation frequency, and an integratorreceiving the output of said synchronous detector means, said integratorproducing a DC output signal representative of the phase shift resultingfrom the distance traveled by said modulation, the output of saidintegrator being connected to said'phase shifting means and to an outputterminal.

2. The ranging system of claim 1 in which the phase shifting meanscomprises a ramp generator connected to said modulating means forgenerating a repeating ramp function having a ramp increasing at asubstantially linear rate, and a comparator connected to said rampgenerator and said integrator, said comparator comparing the integratoroutput voltage with the instantaneous voltage of said ramp to produce analternating signal having a phase determined by the integrator outputvoltage.

3. The ranging system of claim 2 in which the modulating means comprisesan oscillator operating at twice the frequency of said modulation and adividing means connected between said oscillator and said transmittingmeans for reducing oscillator frequency to modulation frequency, and thephase detection means comprises a second dividing means connected tosaid comparator for reducing the .alternating signal from saidcomparator to modulation frequency, said second dividing means alsosquaring the comparator signal along the time axis.

4. The ranging system of claim 3 comprising a phaser connected betweensaid first and second dividing means to establish a predeterminedinitial phase relationship between the outputs of said dividing means.

5. The ranging system of claim 4 comprising a low pass filter connectedbetween said integrator and said comparator, said output terminal beingconnected between said filter and said comparator.

6. The ranging system of claim 5 comprising a second output terminalconnected between said detector and said integrator, said second outputterminal displaying a DC voltage representative of the rate of change ofsaid range.

7. The ranging system of claim 6 comprising a second phase shiftingmeans for shifting the phase of the transmitted modulation in responseto the voltage of the integrator.

8. The ranging system of claim 7 in which the second phase shiftingmeans shifts the phase of the transmitted modulation in the oppositedirection of the first phase shifting means.

9. The ranging system of claim 1 comprising a second output terminalconnected between said detector and said integrator, said second outputterminal displaying a DC voltage representative of the rate of change ofsaid range.

10. The ranging system of claim 1 comprising a second phase shiftingmeans for shifting the phase of the transmitted modulation in responseto the voltage of the integrator.

11. A ranging system for determining the available headway of anautomotive vehicle comprising signal transmitting means located on saidvehicle for directing a carrier signal forward of the vehicle,

modulating means connected to said transmitting means for modulatingsaid carrier signal,

signal receiving means located on said vehicle for receiving reflectionsof the modulated carrier signal from a target located forwardly of saidvehicle, synchronous detector means connected to said modulating meansand said receiving means, said detector means multiplying receivingmeans output by a waveform obtained from said'modulating means toproduce a net DC voltage representative of the phase difference betweenthe modulation and any signal in the receiving means output having thesame frequency as the modulation,

integrator means for integrating detector means output with respect totime, said integrator means producing a DC output voltage that isapplied to an output terminal, said DC voltage at said output terminalbeing representative of available headway, and

phase shifting means connected to said integrator means for shifting thephase of the waveform obtained from the modulating means according tothe output of the integrator means, the output of said phase shiftingmeans being applied to the detector means.

12. The ranging system of claim 11 in which the phase shifting meanscomprises a ramp generator connected to said modulating means forgenerating a repeating ramp function having a ramp increasing at asubstantially linear rate, and a comparator connected to said rampgenerator and said integrator, said comparator comparing the integratoroutput voltage with the instantaneous voltage of said ramp to produce analternating signal having a phase determined by the integrator outputvoltage.

13. The ranging system of claim 12 in which the modulating meanscomprises an oscillator operating at twice the frequency of saidmodulation and a dividing means connected between said oscillator andsaid transmitting means for reducing oscillator frequency to modulationfrequency, and comprising a second dividing means connected between saidcomparator and said synchronous detector means for reducing thealternating signal from said comparator to modulation frequency, saidsecond dividing means also squaring the comparator signal along the timeaxis.

14. The ranging system of claim 13 comprising a phaser connected betweensaid first and second dividing means to establish a predeterminedinitial phase relationship between the outputs of said dividing means.

15. The ranging system of claim 14 comprising a low pass filterconnected between said integrator and said comparator, said outputterminal being connected between said filter and said comparator.

16. The ranging system of claim 15 comprising a second output terminalconnected between said detector and said integrator, said second outputterminal displaying a DC voltage representative of the rate of change ofsaid available headway.

17. The ranging system of claim 16 comprising a second phase shiftingmeans for shifting the phase of the transmitted modulation in responseto the voltage of the integrator.

18. The ranging system of claim 17 in which the second phase shiftingmeans shifts the phase of the transmitted modulation in the oppositedirection of the first phase shifting means.

19. The ranging system of claim 11 comprising a second output terminalconnected between said detector and said integrator, said second outputterminal displaying a DC voltage representative of the rate of change ofsaid available headway.

20. The ranging system of claim 11 comprising a second phase shiftingmeans for shifting the phase of the transmitted modulation in responseto the voltage of the integrator.

21. The ranging system of claim 20 in which the second phase shiftingmeans shifts the phase of the transmitted modulation in the oppositedirection of the first phase shifting means.

1. An electronic ranging system comprising signal transmitting means fordirecting a carrier signal toward a reflecting target, modulating meansfor modulating said carrier signal, signal receiving means for receivingreflections of the modulated carrier signal, and phase detection meansfor determining the phase shift resulting from the distance traveled bysaid modulated carrier signal from said signal transmitting means tosaid signal receiving means, said phase detection means including avoltage controlled phase shifting means connected to the modulatingmeans, synchronous detector means connected to said phase shifting meansand said signal receiving means, said detector means operating at thefrequency of the modulation to detect only received signals having thesame modulation frequency, and an integrator receiving the output ofsaid synchronous detector means, said integrator producing a DC outputsignal representative of the phase shift resulting from the distancetraveled by said modulation, the output of said integrator beingconnected to said phase shifting means and to an output terminal.
 2. Theranging system of claim 1 in which the phase shifting means comprises aramp generator connected to said modulating means for generating arepeating ramp function having a ramp increasing at a substantiallylinear rate, and a comparator connected to said ramp generator and saidintegrator, said comparator comparing the integrator output voltage withthe instantaneous voltage of said ramp to produce an alternating signalhaving a phase determined by the integrator output voltage.
 3. Theranging system of claim 2 in which the modulating means comprises anoscillator operating at twice the frequency of said modulation and adividing means connected between said oscillator and said transmittingmeans for reducing oscillator frequency to modulation frequency, and thephase detection means comprises a second dividing means connected tosaid comparator for reducing the alternating signal from said comparatorto modulation frequency, said second dividing means also squaring thecomparator signal along the time axis.
 4. The ranging system of claim 3comprising a phaser connected between said first and second dividingmeans to establish a predetermined initial phase relationship betweenthe outputs of said dividing means.
 5. The ranging system of claim 4comprising a low pass filter connected between said integrator and saidcomparator, said output terminal being connected between said filter andsaid comparator.
 6. The ranging system of claim 5 comprising a secondoutput terminal connected between said detector and said integrator,said second output terminal displaying a DC voltage representative ofthe rate of change of said range.
 7. The ranging system of claim 6comprising a second phase shifting means for shifting the phase of thetransmitted modulation in response to the voltage of the integrator. 8.The ranging system of claim 7 in which the second phase shifting meansshifts the phase of the transmitted modulation in the opposite directionof the first phase shifting means.
 9. The ranging system of claim 1comprising a second output terminal connected between said detector andsaid integrator, said second output terminal displaying a DC voltagerepresentative of the rate of change of said range.
 10. The rangingsystem of claim 1 comprising a second phase shifting means for shiftingthe phase of the transmitted modulation in response to the Voltage ofthe integrator.
 11. A ranging system for determining the availableheadway of an automotive vehicle comprising signal transmitting meanslocated on said vehicle for directing a carrier signal forward of thevehicle, modulating means connected to said transmitting means formodulating said carrier signal, signal receiving means located on saidvehicle for receiving reflections of the modulated carrier signal from atarget located forwardly of said vehicle, synchronous detector meansconnected to said modulating means and said receiving means, saiddetector means multiplying receiving means output by a waveform obtainedfrom said modulating means to produce a net DC voltage representative ofthe phase difference between the modulation and any signal in thereceiving means output having the same frequency as the modulation,integrator means for integrating detector means output with respect totime, said integrator means producing a DC output voltage that isapplied to an output terminal, said DC voltage at said output terminalbeing representative of available headway, and phase shifting meansconnected to said integrator means for shifting the phase of thewaveform obtained from the modulating means according to the output ofthe integrator means, the output of said phase shifting means beingapplied to the detector means.
 12. The ranging system of claim 11 inwhich the phase shifting means comprises a ramp generator connected tosaid modulating means for generating a repeating ramp function having aramp increasing at a substantially linear rate, and a comparatorconnected to said ramp generator and said integrator, said comparatorcomparing the integrator output voltage with the instantaneous voltageof said ramp to produce an alternating signal having a phase determinedby the integrator output voltage.
 13. The ranging system of claim 12 inwhich the modulating means comprises an oscillator operating at twicethe frequency of said modulation and a dividing means connected betweensaid oscillator and said transmitting means for reducing oscillatorfrequency to modulation frequency, and comprising a second dividingmeans connected between said comparator and said synchronous detectormeans for reducing the alternating signal from said comparator tomodulation frequency, said second dividing means also squaring thecomparator signal along the time axis.
 14. The ranging system of claim13 comprising a phaser connected between said first and second dividingmeans to establish a predetermined initial phase relationship betweenthe outputs of said dividing means.
 15. The ranging system of claim 14comprising a low pass filter connected between said integrator and saidcomparator, said output terminal being connected between said filter andsaid comparator.
 16. The ranging system of claim 15 comprising a secondoutput terminal connected between said detector and said integrator,said second output terminal displaying a DC voltage representative ofthe rate of change of said available headway.
 17. The ranging system ofclaim 16 comprising a second phase shifting means for shifting the phaseof the transmitted modulation in response to the voltage of theintegrator.
 18. The ranging system of claim 17 in which the second phaseshifting means shifts the phase of the transmitted modulation in theopposite direction of the first phase shifting means.
 19. The rangingsystem of claim 11 comprising a second output terminal connected betweensaid detector and said integrator, said second output terminaldisplaying a DC voltage representative of the rate of change of saidavailable headway.
 20. The ranging system of claim 11 comprising asecond phase shifting means for shifting the phase of the transmittedmodulation in response to the voltage of the integrator.
 21. The rangingsystem of claim 20 in which the second phase shifting means shifts thephase of the transmitted modulation in the opposite direction Of thefirst phase shifting means.